The present invention relates to group delay equalizers, and more particularly to such equalizers as used in the I.F. portion of a UHF television transmitter.
Klystrons having multiple output cavity resonators are typically used in UHF television transmitters. The cavities cause group delay to vary across the passband of the desired channel, thereby distorting the television signal. In particular, the group delay characteristic has a double humped response. This causes "spiking" at abrupt luminance signal transitions, and causes the chroma signal to have a different group delay from that of the luminance signal.
Typical prior art compensation circuits use a circuit, usually located in the transmitter I.F. section, that has the inverse of klystron group delay characteristic. Since at the IF frequency (about 45 MHz) a passive group delay equalizer with a "flat" frequency response cannot be made due to unattainably high required coil Q, the prior art uses an active equalizer comprising two I.F. paths. A series tuned circuit having a transfer function of T(j.omega.) is in the first path, while the second path is a direct connection (except for a possible all-pass delay compensator network). The signals in the two paths are subtracted, with the signal from the tuned circuit first path having twice the amplitude of the signal in the direct connection second path. The resulting transfer function is 2 T(j.omega.)-1, which can properly equalize the group delay.
Such an equalizer has several disadvantages. A group delay adjustment in the tuned circuit first path causes a signal level change at the output of the subtractor. This requires an amplitude control to be added, which control must be adjusted as the group delay control is changed. Also, an active circuit such as an emitter follower is typically used in the first path for isolation purposes, which emitter follower must have a stable gain.
It is therefore desirable to provide a delay compensation network that is easy to adjust and is stable.